This invention relates to sized fibers for use as reinforcement in composites, and more particularly to reinforcing fibers coated on the surface with a sizing composition comprising polyamide-amic acid, amide-imide polymer, amide-imide copolymer, amide-imide phthalamide copolymer or mixtures of these materials (hereinafter collectively referred to as amide-imide polymers) and to composites prepared from these sized fibers. The sized fibers may be used to prepare composites with amide-imide polymers as the matrix resin having excellent toughness properties and enhanced thermal properties for extended use at temperatures in excess of 450.degree. F.
Carbon fiber composites wherein carbon fibers are included in a synthetic resin matrix are well known in the art as is the use of other reinforcing fibers. The function of the resin matrix is primarily to give the composite the required external dimension and shape, while the reinforcing fibers provide the composite with the required strength and stiffness.
Improved bonding between, for example, carbon fibers and matrix resins in general has been accomplished in the past by a number of techniques such as by plating the carbon fibers with various metals including tantalum, metal carbides and nitrates, and by etching the surface of the carbon fibers with oxidizing agents such as air, ozone, concentrated nitric acid and chromic-sulfuric acid. Block and graft copolymers have also been used to improve bonding between the carbon fibers and the matrix as illustrated by U.S. Pat. No. 3,855,174. Carbon fibers have also been coated with a variety of sizing agents such as epoxy resins for a number of reasons, such as to prevent snagging and fraying of the carbon fibers as illustrated by U.S. Pat. Nos. 3,373,984; 3,806,489; 3,908,042; 3,914,504; 3,953,641; 3,971,669; and 4,145,472, and British Patent Specification No. 1,195,219. U.S. Pat. No. 4,364,993 uses a polyisocyanate or a sorbitol polyglycidyl ether as sizing agents for carbon fibers.
Amide-imide polymers and copolymers are a relatively new class of organic compounds known for their solubility in nitrogen-containing organic solvents when in the largely polyamide form. In the past, the major application of these amide-imide polymers has been as wire enamels. This is illustrated in U.S. Pat. Nos. 3,661,832 (1972), 3,494,890 (1970) and 3,347,828 (1967). Amide-imide polymers and copolymers have also been found useful for molding applications as shown in U.S. Pat. Nos. 4,016,140 (1977) and 3,573,260 (1971). U.S. Pat. Nos. 4,136,085 (1979), 4,313,868 (1982), and 4,309,528 (1982) are incorporated herein by reference. These polyamide-imides are known for their outstanding mechanical properties. Filled polyamide-imide resins and composites are also known in the art. The agents commonly used for sizing fibers and fillers tend to volatilize or decompose at the very high temperatures used to process amide-imide matrix resins, forming volatiles which cause voids in the final composite structure. Additionally, most of these sizing agents do not provide acceptable binding between the fiber and the amide-imide polymer matrix. More recently, in U.S. Pat. No. 4,467,011, there was disclosed a method for improving the melt flow of amide-imide polymers by blending the resins with amorphous polyamides. These amorphous polyamides were also shown to be useful as sizing compositions for use with glass fibers, chopped fiber fillers, and various reinforcing fabrics in forming reinforced and filled polyamide-imide resins. Although these filled compositions have good processability and are readily injection-molded, thick-walled sections and structures having thick cross-sections exhibit a tendency toward cracking and void formation which severly limits the possible applications for such compositions.
Accordingly, there is a need for fiber sizing agents having sufficient thermal stability to withstand amide-imide polymer processing temperatures, providing improved binding between the fiber and the amide-imide polymer matrix while avoiding or reducing the tendency toward forming voids, cracks and similar defects in the final composite.